A lubricating oil base oil is required to exert its performance stably for a long period of time, i.e., to have low volatility, excellent heat and oxidation stability and low-temperature startability (low-temperature fluidity), and a high viscosity index (wide range). In particular, it is not too much to say that a lubricating oil base oil having characteristics of low viscosity and low volatility is an ultimate aim.
Along with improvements in performance of audio-visual and office automation equipment, a small spindle motor used in a rotating part in the equipment has been strongly required to be refined so as to achieve speed-up and electrical power saving. Therefore, a bearing used in a rotation-supporting part has been constantly required to achieve low torque. Meanwhile, particularly recently, the bearing has been required to have performance applicable to various environments (temperatures) in consideration of use as a mobile device. As a factor having an effect on the torque of the bearing, there are given a bearing clearance and a shaft diameter. In particular, the viscosity of a lubricating oil in a low-temperature environment is a major factor.
In general, a lubricating oil having a lower viscosity tends to easily evaporate. When an amount of the lubricating oil decreases due to evaporation, the bearing is judged to come to the end of its life because of an inappropriate oil film pressure and significantly lowered rotation accuracy. Therefore, an evaporation characteristic of the lubricating oil is an important characteristic which affects durability of the bearing. Accordingly, in lubrication of a sliding bearing such as a fluid dynamic pressure bearing, an oil-impregnated porous bearing, or a dynamic pressure-type oil-impregnated porous bearing, it is necessary to select a lubricating oil which has a low viscosity, does not cause an increase in the viscosity even in a low temperature range, and has a relatively excellent evaporation characteristic. In many cases, an ester-based lubricating oil is used.
Like other lubricating oils, an ester oil tends to have a lower evaporation characteristic as the viscosity becomes lower. Therefore, to reduce the torque of the bearing, even when an ester oil having a lower viscosity than that of a conventional one is selected, the evaporation characteristic is impaired, resulting in a reduction in durability of the bearing. In addition, even when the oil has a low viscosity at ordinary temperature, a rapid increase in torque or stopping of devices may occur when the viscosity increases drastically or the fluidity is lost in a low temperature range.
Particularly, in recent years, hard disks are often installed in home electronics and may be used at low temperature in many cases. Therefore, in order to ensure stable driving, a low viscosity in a low temperature range has been strongly required. Many lubricating oil base oils have been proposed to satisfy such properties. However, in the present circumstances, the oils do not satisfy the low viscosity and low volatility which are ultimate aims although the oils satisfy the properties to some extent.
The low viscosity and low volatility contradict each other. For example, when the viscosity is reduced without changing its structure, the molecular weight decreases, naturally resulting in an increase in volatility. As means for solving such defects, an ester-based base oil having a low viscosity and relatively excellent evaporation property is used.
Patent Literature 1 discloses a lubricating oil composition including, as a base oil, a diester obtained from a linear divalent alcohol having 6 to 12 carbon atoms and a branched saturated monovalent fatty acid having 6 to 12 carbon atoms.
However, according to the conventional technology, a lubricating oil having low-viscosity property can be obtained by appropriately selecting an alcohol and a fatty acid. However, in the case of a diester having a viscosity at 40° C. of 10 mm2/s or less, the evaporation amount becomes larger as its molecular weight becomes lower. Further, the evaporation occurs concurrently owing to a uniform molecular weight, and hence the durability may drastically deteriorate from a certain condition. This is because many of esters have symmetrical chemical structures. That is, the limiting point is clear because of a single composition, and the evaporation may cause sudden stopping of the motor. This is probably because, in a combination of 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol with 2-ethylhexanoic acid and 3,3,5-trimethylpentanoic acid, which is considered by the above-mentioned conventional technology to be particularly suitable, the viscosity index is small because a ratio of components having branched carbon structures is large based on the molecular weight, and the viscosity becomes particularly high at low temperature, resulting in an adverse effect on driving property of the motor under a usual environment. In addition, this is probably because, as the ratio of the branched structures in the diesters becomes larger, the evaporativity becomes larger.
Patent Literature 2 discloses a lubricating oil composition which contains: as a major component, an ester synthesized from a monovalent alcohol having 8 carbon atoms and a divalent carboxylic acid having 6 carbon atoms; and, at a concentration of 1 to 5 wt %, a diester which is different from the major component, has a kinetic viscosity at 40° C. of 10 mm2/s or more, and has a total of 23 to 28 carbon atoms in its molecule, and a fluid bearing unit using the lubricating oil composition.
Patent Literature 3 describes a lubricating oil base oil containing, as a major component, a diester compound or a triester compound synthesized from a divalent or trivalent carboxylic acid having 9 or less carbon atoms and a monovalent glycol ether such as an alkylene glycol monoalkyl ether having 3 to 25 carbon atoms.
However, the lubricating oils or lubricating oil base oils described in the literatures do not fully satisfy the requirements of low viscosity and low volatility.